This invention relates to delay line digital code detectors and to digital delay line detectors which have particular use in detecting valid replies from transponders operating in the air traffic control radar beacon system.
The present air traffic control radar beacon system includes a network of ground beacons which generally transmit interrogation signals into the air space comprising their field of interest and airborne transponders which respond to received interrogations by transmitting a code indentifying the aircraft or its altitude, depending on the specific interrogation signal transmitted by the ground station. As presently constituted, a transponder response consists of a pair of bracket or framing pulses spaced a predetermined time apart, which bracket pulses enclose 13 equally spaced information positions, each of which may or may not be occupied by a pulse depending upon the informational content of the response. In addition, the response can include an indentification pulse which trails the final framing pulse by a predetermined time. Responses received at a ground beacon interrogator or other receiving station are detected and the informational content extracted therefrom for air traffic control purposes. Since in the present air traffic control radar beacon system interrogations are transmitted in a narrow beam radiated from the ground station, responses received from a particular interrogation identify the azimuth of the responding transponder with respect to the station.
Considering the large number of beacon interrogators in many areas and the large number of aircraft having transponders which are operating in the field of interest, it is not surprising that a large number of unsynchronized replies or responses from ambiguous azimuths are received at each interrogator. Extracting only valid responses from the large number of responses received and suppressing garbled information obviously constitutes a problem.
It can be seen that it is highly desirable and often imperative that garbled responses be inhibited. By a garbled response is meant the simultaneous arrival at a receiving station of responses from two or more separate transponders so as to interfere with the extractions of information from the various responses. It is also important that phantom replies be suppressed. Phantom replies are apparent replies to interrogations which manifest themselves at the receiving station as properly spaced framing pulses but which in actuality are comprised of a first framing pulse received from one transponder and consisting either of an actual framing pulse or an informational pulse from that transponder's response and a second apparent framing pulse which consists of either an actual framing pulse or an informational pulse from the other transponder's response.